Utility meters are used for billing services provided by public utilities such as power, gas and water. For example, kilowatt-hour meters, located on customer premises, include detachable meter units for measuring and recording electric power consumption by the customer. Typically, an induction-type kilowatt-hour meter is provided at each customer location. The induction type kilowatt-hour meter operates on the same principle as that of a rotating magnetic field in an squirrel-cage induction-type motor. Electric power service is routed through the meter in a manner causing a metallic disk to revolve at a rate proportional to power consumption. Disk rotation is counted and recorded mechanically using a mechanical kilowatt-hour register and/or electronically with data stored in a conventional semiconductor memory. Meter reading personnel periodically inspect each customer site and record utility meter readings, either by hand or using an electronic probe to retrieve data stored in solid state memory. Billing information is generated based on the data collected by the meter reading personnel.
To increase data collection efficiency and reliability, utility meters are now available which include interface equipment to permit remote interrogation of the meter and transmission of usage data. Connectivity between utility meters located at remote customer sites and a central billing facility can be provided using various media including signals transmitted on power lines, dedicated signaling lines, the public telephone switched network (PTSN) and radio frequency (RF) transmissions.
As a result of increased utilization of automatic remote reading of utility meters, there has been a decreased frequency of on-site inspection of metering equipment, providing an increased opportunity for undetected tampering with the metering equipment. For example, most single phase and polyphase electricity meters in the United States are socket mounted. Most common methods of tampering with such electricity metering installation involves removing the meter from the meter socket. Once removed, a single phase meter, for example, can be reinstalled upside down resulting in reverse rotation of the internal meter disk and register dials which record cumulative energy consumption.
Earlier generations of conventional induction type kilowatt-hour meters employed a mechanical technique to sense the installation of a meter and a counter sealed beneath the meter cover to record and display the number of times the meter had been removed and reinstalled. Later generations of induction meters employed orientation sensitive switches (tilt switches) and logic elements to sense common tampering techniques.
Although tilt switches have proven effective at sensing removal and reinstallation of meters when reinstalled upside down, these techniques are not able to reliably sense meter removal from a socket when a reasonable attempt is made to maintain the meters in a normal attitude. If the tilt switch is made overly sensitive, then the system will falsely report tampering in response to normal events such as a branch blowing against the meter or other foreign object inadvertently striking the meter or supporting structures. Conversely, if the tilt switch is insufficiently sensitive, then a person observing reasonable precautions can remove the meter from its socket without activating the tamper detection system.
Another prior art tamper detection system is U.S. Pat. No. 5,331,412 to Farmer, et al. which issued Jul. 19, 1994. Farmer discloses a circuit which monitors certain signals coming from a single dwelling interdiction unit, and if the signals are interrupted, removes service from the premises. In order for service to reinstituted to the subscriber, DC power from the single dwelling interdiction unit should be communicated to a tap microprocessor.